Breitbart's father in-law, former game show host, Orson Bean, said Breitbart was walking near his house in Los Angeles when he collapsed. He'd reportedly had heart problems in the past.

Breitbart's website says he died "Unexpectedly from natural causes."

But Rosie O'Donnell tweeted: "it's quite odd, no? What is 'natural causes' for a 43-yr-old man?"

There was an animated exchange in what is thought to be his last TV appearance just a couple of weeks ago. Durring the appearance on Current TV's The Young Turks, he was discussing the Occupy Wall Street movement, when he joked about his blood pressure.

"Sorry if my blood pressure is a tad high," said Breitbart.

Breitbart made his name when he broke the Anthony Weiner photo sexting scandal.